Biological clocks of eukaryotic organisms establish temporal programs for the occurrence of fundamental biological processes, e.g., metabolism, development, and reproduction. The light:dark cycle is a basic environmental rhythm that affects both circadian and seasonal (photoperiodic) clocks. Many circadian and seasonal clocks are located in the central nervous system and involve neuroendocrine systems. It is by the action of neuroendocrine products that these neural clocks affect an organism, and perturbations of clock function can have profound adverse effects on biological function. Vertebrate models, because of their complexity, have proven difficult subjects for the study of the cellular and molecular organization of photoperiod-cued clocks and their temporal control of neurosecretion. Invertebrates, especially insects, are promising alternative models because of the relative simplicity of their clock/neuroendocrine systems. In the tobacco hornworm, Manduca sexta, a clock(s) and its photoreceptor control the timing of the release of a cerebral peptide, big prothoracicotropic hormone (big PTTH), that drives insect development; the clock may reside within the neurons that produce this peptide. Since in this species certain populations of cerebral neurosecretory neurons, and possibly endocrine cells, have been identified as clock-controlled, it is proposed that circadian and photoperiodic clock regulation of these identified cell types is a function of the big PTTH neurons. The research proposed will determine if the big PTTH neurosecretory cells do function as a clock and will begin to elucidate the structural basis of this clock and its intercellular control of developmental functions. This will be accomplished by: 1) demonstrating that the big PTTH neurons possess an endogenous circadian and/or photoperiodic clock; 2) establishing that an identified organelle unique to the big PTTH neurons is part of the clock; and 3) establishing that the clock regulates the synthesis and packaging peptidergic products of the PTTH neurons. A multidisciplinary approach, e.g., physiological, cell biological, neurophysiological, immunological, and molecular biological, will be utilized to accomplish these objectives. The results of this research should provide unique information about the molecular and cellular organization of a biological clock and the mechanisms by which it exerts its effects.